Fluid spring shock absorbers

ABSTRACT

A shock absorbing and energy dissipating device suitable for an aircraft landing gear comprising: I. A CYLINDER; II. A PISTON ROD, OF SMALLER DIAMETER THAN THE INTERNAL DIAMETER OF THE CYLINDER, MOVABLE INTO AND OUT FROM THE CYLINDER THROUGH A SEAL AT ONE END OF THE CYLINDER, THE OTHER END OF THE CYLINDER, WHICH IS THE END INTENDED TO BE LOWERMOST IN USE, BEING CLOSED; III. A PISTON CARRIED BY THE PISTON ROD AND SEALINGLY ENGAGING THE CYLINDER; IV. A VOLUME OF LIQUID IN THE CYLINDER; V. MEANS FOR RESTRICTING FLOW OF LIQUID FROM ONE SIDE OF THE PISTON TO THE OTHER AS THE PISTON MOVES IN THE CYLINDER TOWARDS SAID OTHER END OF THE CYLINDER; VI. A CONTAINER, CONTAINING A VOLUME OF GAS IN DIRECT UNRESTRICTED COMMUNICATION WITH LIQUID AT THE SAID ONE END OF THE CYLINDER; AND VII. FLOW-PASSAGE MEANS ADAPTED TO PERMIT UNRESTRICTED FLOW OF FLUID FROM ONE SIDE OF THE PISTON TO THE OTHER WHEN THE DEVICE IS IN ITS FULLY-EXTENDED POSITION; VIII. THE LIQUID AND THE GAS BEING PRESSURIZED IN THE FULLY EXTENDED POSITION OF THE DEVICE TO A VALUE ADAPTED TO OVERCOME FRICTIONAL FORCE ASSOCIATED WITH THE DEVICE WHEN AN EXTERNAL LOAD THEREON IS REMOVED THEREBY TO EXTEND THE DEVICE TO ITS FULLY-EXTENDED POSITION; AND IX. THE VOLUME OCCUPIED BY GAS IN SUCH POSITION BEING LESS THAN THE VOLUME OF THE PISTON ROD INTRODUCED INTO THE CYLINDER BY MOVEMENT OF THE ROD FROM THE FULLY-EXTENDED POSITION TO THE FULLY-CONTRACTED POSITION.

United States Patent 191 Savery 45] July 24, 1973 FLUID SPRING SHOCKABSORBERS [75] Inventor: Ralph Savery, Gloucester, England [73]Assignee: Dowty Rotol Limited, Gloucester,

England [22] Filed: Aug. 2, 1971 [21] Appl. No.: 168,211

[ 30 Foreign Application Priority Data Aug. 15, 1970 Great Britain..39440/70 [52] YQIIT267T6TT [51] Int. Cl F16 5/00 [58] Field of Search267/65 R, 65 A, 65 B, 267/64 R [56] References Cited UNITED STATESPATENTS 2,107,494 2/1938 Onions et al 267/64 R 3,626,864 l2/l97l Wiebe267/64 A 2,909,368 10/1959 Taylor... 267/64 A 3,348,835 10/1967 Casey267/65 R Primary Examiner-James B. Marbert Attorney-Irvin S. Thompsonand Robert J. Patch [57] ABSTRACT A shock absorbing and energydissipating device suitable for an aircraft landing gear comprising:

i. a cylinder;

ii. a piston rod, of smaller diameter than the internal diameter of thecylinder, movable into and out from the cylinder through a seal at oneend of the cylinder, the other end of the cylinder, which is the endintended to be lowermost in use, being closed;

iii. a piston carried by the piston rod andv sealingly engaging thecylinder;

iv. a volume of liquid in the cylinder;

v. means for restricting flow of liquid from one side of the piston tothe other as the piston moves in the cylinder towards said other end ofthe cylinder;

vi. a container, containing a volume of gas in direct unrestrictedcommunication with liquid at the said one end of the cylinder; and

vii. flow-passage means adapted to permit unrestricted flow of fluidfrom one side of the piston to the other when the device is in itsfully-extended position;

viii. the liquid and the gas being pressurized in the fully extendedposition of the device to a value adapted to overcome frictional forceassociated with the device when an external load thereon is removedthereby to extend the device to its fully-extended position; and

ix. the volume occupied by gas in such position being less than thevolume of the piston rod introduced into the cylinder by movement of therod from the fully-extended position to the fully-contracted position.

12 Claims, 3 Drawing Figures United States Patent 1 [111 3,747,913Savery 1 July 24, 1973 PATENTED JUL24 I973 SHEET 1 BF 3 FLUID SPRINGSHOCK ABSORBERS This invention relates to a combined shock absorbing andenergy dissipating device suitable for aircraft landing gear. It is ageneral requirement for all aircraft components to be as light and assmall as possible to carry out their intended functions. The basicfunctions of a combined shock absorbing and energy dissipating deviceare firstly to support the weight of the aircraft when on the groundwith a damped resilience to facilitate taxying over the ground and,secondly, to absorb and dissipate the kinetic energy of the verticalcomponent of aircraft velocity when landing.

In the recent past, combined shock absorbing and energy dissipatingdevices for aircraft have fallen into two classes:

1. The oleo-pneumatic piston-and-cylinder type in which shock absorbingis effected by compression of preloaded gas and energy dissipation iseffected by a hydraulic dash-pot device.

2. The liquid spring piston-and-cylinder type in which the liquid of ahydraulic dash-pot is subjected to elastic compression so that thesamevolume of liquid provides both shock absorption and energy dissipation.

Class 1 devices suffer from the disadvantage of larger size andsubstantial weight since they invariably include separatepiston-and-cylinder devices for the gas and the liquid, and also operateat comparatively low pressures.

Class 2 devices improve on the Class 1 devices by not requiring spacefor shock absorbing gas, but on the other hand the liquid is subjectedto substantially higher pressures than in Class 1 devices and the singlecylinder is therefore somewhat heavier to withstand the higherpressures. Thus there is little weight difference between Class 1 andClass 2 devices, but a Class 2 device shows substantial space saving.

The principal object of the present invention is to provide a combinedshock absorbing and energy dissipating piston-and-cylinder device inwhich one volume of liquid performs shock absorbing and energydissipating functions and in which there is substantial space and weightreduction as compared with either of the two known classes of device setout above.

A practical disadvantage of a Class 2 device is that it cannot functionequally well over a range of atmospheric temperatures such as arenormally encountered by an aircraft since with increasing temperaturethe volume of liquid within the device will expand or tend to expand andthereby increase the liquid pressure to the extent to impair the shockabsorbing function, and with reduced temperature the volume of liquidwill contract and prevent full extension of the device.

A further object of the present invention is to provide a combined shockabsorbing and energy dissipating de vice which uses one volume of liquidfor shock absorbing and energy dissipating functions and in whichtemperature variations as normally encountered in the atmosphere willnot adversely effect these functions in a substantial way.

In accordance with the present invention a shock absorbing and energydissipating device suitable in an aircraft landing gear comprises:

i. a main container having a single aperture intended to be uppermost inuse ii. a plunger movable into and out of the aperture in the'maincontainer iii. a sea] at the said aperture to sealingly engage againstthe relatively movable surface of the plunger iv. a volume of liquidwithin the main container v. a second container containing gas atpressure vi. a restricted flow passage between the main container andthe second container for restricting liquid flow between them vii. anunrestricted flow passage for unrestricted gas flow in a generallyupward direction from substantially the whole volume of the maincontainer to the second container open when the plunger is in its fullyextended position.

The invention achieves its main object by the increased compressibilityof the total fluid in the shock absorber and by the fact that the shockloading pressures are confined to the liquid. Preferably, the liquidpressures generated in the shock absorber are arranged not to exceedabout 40,000 p.s.i., this pressure representing the maximum pressure atwhich the main container in the form of a cylinder may be constructedwithout pre-stressing and also the maximum pressure at which the dynamicseal is capable of efficient longwearing operation. The main containermay be a cylinder and the plunger may be a piston rod carrying at itsinner end a piston sliding in the cylinder.

As compared with a conventional Class 2 device limited to a maximum of40,000 p.s.i. the invention enables the piston-and-cylinder to besuitably reduced in diameter whilst the piston rod diameter is eitherunaltered or slightly larger. This reduction in piston diameter iscompensated by altering therestrictor or valve means controlling flowbetween the sides of the piston so as to increase the shock loadingpressure towards the static loading pressure. It will be appreciatedthat static loading pressure acts effectively only over the crosssectionof the piston rod whereas as the shock loading pressure acts jointly onthe piston and the piston rod. Therefore a lower shock loading pressurecan represent a higher thrust in the piston rod than would be obtainedfor the higher static pressure. Thus as compared with a Class 2 devicethe energy dissipation characteristics may be identical.

The presence of the gas increases the compressibility of the total fluidcontent of the cylinder so that the total extended fluid volume needs tobe only about five times the reduction of fluid volume during piston rodmovement into the cylinder over a full working stroke. A Class 2 devicerequires a liquid capacity of at least ten times the reduction of liquidvolume for maximum piston rod stroke if moderately high pressures onlyare used. Where the maximum static pressure is 40,000 p.s.i., the pistonrod may be madeof tubular form, the volume available within it beingsufficient to accommodate the volume of gas.

There needs to be no means to separate the gas from the liquid, providedthat at the pressurised pressure the gas will not dissolve to anysubstantial extent in the liquid thus ensuring that pressure isavailable to extend the piston rod when the device is unloaded. At thehigher pressures within the cylinder generated by movement of the pistonrod into the cylinder the gas may dissolve in the liquid, but thisproduces no real difficulty since the volume of liquid'present is quiteadequate on its own to give the shock absorption and energy dissipationthat may then be necessary.

A combined shock-absorbing and energy dissipating device suitable for anaircraft undercarriage is illustrated in the drawings, in which FIG. 1,accompanying the Specification, is a longitudinal section,

FIG. 2 accompanying the Specification, is a detail of FIG. 1 on anenlarged scale, and

FIG. 3 accompanying the Specification, is a diagram showing the effectof different proportions of liquid and gas under compression.

In FIG. 1, a cylinder member 11 has one closed end 12 formed with apivotal attachment eye 13, while the other end is closed by ascrew-threaded ring 14 which supports a packing gland assembly 15 of thewell known kind having an unsupported area. A piston rod 16 extendingslidably through the ring 14 and the packing gland assembly 15, has itsouter end portion formed with an attachment eye 17. A piston 18 carriesa piston ring 19 which slidably engages the bore 21 of the cylinder 11,and it is formed with a skirt portion 22 which is screw-threadedlyengaged with the inner end portion of the piston rod 16.

The cylinder 11 and the piston 18 and rod 16 define a lower chamber 23below the piston, an upper chamber 24 above the piston and a cavity 34within the piston rod 16. A circular row of holes 25 are formed in thepiston. A lower orifice plate 26 and an upper orifice plate 27 are urgedtowards retaining rings 28 and 29 by light springs 31. These springs arelocated in some of the holes 25, while leaving the other holesunobstructed particularly for passage of gas. The lower and upperorifice plates 26, 27 are alternatively movable by fluid flow, when thepiston 18 moves in the bore 21, in one direction or the other to seat onthe holes 25 so that restrictor holes in the plate act to restrict flowof liquid from one side of the piston to the other to provide energydissipation. Very low piston speeds will not cause movement of theorifice plates.

The skirt portion 22 is engageable with the gland assembly 15 to limittelescopic extension of the cylinder and piston members. Slots 32 in theskirt portion 22 and holes 33 in the piston rod 16 connect the upperpart of the upper chamber 24 to the cylindrical cavity 34 which isformed within the piston rod 16. A second connection is provided byholes 35 in the skirt portion 22 and in the piston rod 16 at a positionnearer to the piston 18.

A relief valve is provided having a valve orifice 36 in I the piston 18,and a piston-like valve member 37 which is slidable in a bore 38 withinthe piston, the valve member 37 having a central projection 42 whichcloses the orifice 36 under the load of a spring 39. A discharge orifice41 leads from the bore 38 on the discharge side of the orifice 36 to thechamber 24. The discharge orifice 41 may provide a restriction acrosswhich the valve discharge. develops a pressure difference acting on thevalve member 37, whereby the relief valve, once open, tends to remainopen during shock absorber compresmm.

A charging nozzle 43 fitted to the closed end 12 of the cylinder 11,communicates with the chamber 23.

To put the shock absorber in readiness for use, it is filled withhydraulic liquid in the fully extended condition so that the entirechamber, i.e. the lower chamber 23, the upper chamber 24 and the cavity34 are full of liquid. With a drain plug, not shown, adjacent thecharging nozzle 43 open, the shock absorber is telescopically contractedso that the chamber is reduced in volume by a predetermined percentageof the maximum volume. The drain plug is then closed and nitrogen, orother inert gas, is introduced so that the shock absorber extends andbecomes charged to a predetermined pressure. This pressure is initiallyhigher than the required pre-load pressure but it will fall as gaspasses into solution in the hydraulic liquid until there is equilibriumbetween the dissolved gas and the remaining free gas. At this stage ofequilibrium, the pressure in the chamber will be substantially equal tothe required pre-load pressure. This condition is more rapidly achievedby operating the shock absorber so that the liquid and gas become mixed.

FIG. 3 shows the relationship between static pressure and percentagecompression of a fluid contained in a shock absorber for differentproportions of liquid and gas. The pressure is measured in pounds persquare inch (p.s.i.) while the compression is the reduction of fluidvolume expressed as a percentage of maximum fluid volume, that is, thevolume at full extension of the shock absorber.

Curve A shows the relationship for a fluid filling consisting wholly ofa suitable mineral oil, as in a liquid spring. The compression at 30,000p.s.i. is 8.3 per cent.

Curve B shows the relationship for a fluid filling having 94 per cent ofliquid and 6 per cent of free gas at 1500 p.s.i. in equilibrium with thegas in solution. In an aircraft at rest on the ground, the staticloading on an undercarriage incorporating the shock absorber willproduce a pressure in the shock absorber considerably in excess of thepre-load pressure, whereby over a period of time all the free gas entersinto solution. The relationship between pressure and compression thenfollows the broken line curve Bs.

Curves C and D illustrate the relationships for fluid fillings having 10per cent and 15 per cent respectively of free gas, while the broken linecurves Cs and Ds illustrate the respectively related curves when all thegas has entered into solution due to the undercarriage standing on theground under load.

Considering, for example, a shock absorber containing 15 per cent gas at1500 p.s.i., the gas will pass into solution when the aircraft isstanding on the ground. During taxying and prior to takeoff of theaircraft, the relationship between the pressure and the compression willsubstantially follow the broken line curve Ds if the effect of thedamping by the orifice plates 26 and 27 is ignored.

When the landing gear is retracted, the load on it is removed and thegas pressure will now act to extend the shock absorber fully against anyfrictional resistance due for example to the seal 15 even though duringretraction the shock absorber may be located with chamber 23 uppermost.Some of the gas will come out of solution to maintain the pre-loadpressure. When the landing gear is lowered prior to landing, into apoistion where the chamber 23 is lowermost, any free gas in the lowerchamber 23 will flow through the open holes 25 in the piston 18, sincethe orifice plates 26 and 27 will be held clear of the holes 25 by thesprings 31.

Any gas in the upper chamber 24 will flow through the slots 32 and theholes 33 into the cavity 34, while liquid thus displaced from the cavitywill flow through the lower holes 35 into the upper chamber 24. Slots 32and holes 33 must therefore be sufficiently large for unrestricted gasflow. On touch down, the lower chamber 23 is substantially filled withhydraulic liquid with a minimum of gas in solution.

The relationship between pressure and compression will, assuming theoriginal gas proportion is per cent, during the initial landing shock atleast, follow the curve C if the effect of the damping by the orificeplates 26 and 27 isignored, whereby there is continuously increasingfluid pressure resistance to compression ofthe shock absorber from thefully extended condition.

Damping or energy dissipation on initial landing results from the rapidmovement of the piston 18 into the chamber 23 which will immediatelyseatthe plate 26 on to the passages 25 so that the only excape passageis through restrictors 20 into chamber 24. Since the piston has asmaller diameter than the equivalent Class 2 device the pressuregenerated in chamber 23 is higher to provide the same shock load and therestrictors 20 are of suitable size to provide such pressures. Therelief valve 37 is of substnatially identical structure with a reliefvalve for use on an equivalent Class 2 device with the exception thatthe orifices 36 and 41 are smaller. The size of orifice 36 inconjunction with the preloading of spring 39 is such that a pressuredifference of about 20,000 p.s.i. between chambers 23 and 24 isnecessary to lift valve member 37. Liquid flow from chamber 23 thenflows in succession through orifices 36 and 411, the pressure dropthrough orifice 41 acting on the full area of valve 37 to maintainitopen. This pressure drop, which is the pressure difference betweenchambers 23 and 24, is considerably lower than the pressure originallyneeded to lift valve 37 and can only occur after the piston rod hasentered substantially into the cylinder and the static pressure hasitself become quitehigh to resist further shock absorber compression.Thus during landing the pressure in chamber 23 could be quite high overthe majority of the shock absorber stroke due to the combined shockabsorbing and energy dissipating action and for preference this pressureis designed not to exceed about 30,000 p.s.i. The structure of cylinder11 may then have a reasonably thin wall and avoid the need forpre-stressing.

In the described embodiment the percentage of gas is arranged to beabout 15 per cent of the total extended fluid volume whilst the volumeof piston rod entering the cylinder over its full stroke is about 20 percent of the total extended fluid volume. In the equivalent Class 2device if pressure is to be kept down to 30,000 p.s.i. the volume ofpiston rod entering the cylinder for full stroke can only be about 8 percent of the extended liquid volume. Thus it will be seen that thepresent invention enables a very substantial reduction in cylinder size,as compared with a Class 2 device, to be obtained.

The described embodiment also incorporates recoil damping. During recoilof the shock absorber the liquid pressure in chamber 23 acting over thefull area of piston 18 will act to urge the piston rod outwardly againstthe liquid pressure in chamber 24 so that liquid will flow from chamber23 to chamber 24 to seat plate 27 on the passages 25. Restrictors 20 inplate 27 will then restrict such flow and damp the recoil.

Yet another advantage .over a Class 2 device is that the shock absorbercan extend the landing gear fully at low temperatures when relieved ofground load, where a filling of hydraulic liquid alone would otherwisebe unable to effect full extension of the effect of different thermal.expansion coefficients of the shock absorber body and the liquid.

When the temperature of the present shock absorber falls, the compressedgas expands to compensate for reduction of liquid volume and ensure fullextension of the landing gear, such full extension beingessential foreffective energy dissipation over the whole stroke during landing.

In the described embodiment the holes 25 in the piston are provided ofsuch size that when the'piston is static within the cylinder gas mayeasily pass through towards the chamber 34. In an alternative embodimentthe holes 25 may be replaced by a groove or grooves in the wall of thecylinder 21 at the position occupied by the piston in the fully extendedposition of the piston rod. The piston includes one or more permanentlyrestricted holes to dampen movement of the piston in the cylinder andthe relief valve 37. The groove or grooves will permit unrestricted flowof gas bubbles around the piston when .in the fully extended piston rodposition thus allowing gas which comes out of solution when the shockabsorber is unloaded and when the cylinder is in its lowermost positionto pass the piston freely into the chamber 34.

I claim:

1. A shock absorbing and energy dissipating device suitable for anaircraft landing gear comprising:

i. a main container having a single aperture intended to be uppermost inuse ii. a plunger movable into and out of the aperture in the maincontainer iii. a seal at the said'aperture to sealingly engage againstthe relatively movable surface of the plunger iv. a volume of liquidwithin the main container v. a second containercontaining gas atpressure vi. a restricted flow passage between the main container andthe second container for restricting liquid flow between them vii. anunrestricted flow passage for unrestricted gas flow in a generallyupward direction from substantially the whole volume of the maincontainer to the second container open when the plunger is in its fullyextended position. I

2. A shock absorbing and energy dissipating device as claimed in claim 1wherein the piston rod is hollow and part thereof forms the secondcontainer.

3. A shock-absoring and energy dissipating device as claimed in claim 2wherein the piston rod has abutment means adapted to determine theoutermost position of the piston rod .at which the piston is spaced fromthe said seal, the piston rod between the piston and the seal havingaxially-spaced holes each connecting the hollow piston rod to thecylinder at least the hole nearer to the seal forming part of saidunrestricted flow passage.

4. A shock absorbing and energy dissipating device as claimed in claim 1including valve means responsive to movement of the plunger to close thesaid unrestricted passage.

5. A shock absorbing and energy dissipating device as cliamed in claim 4wherein the unrestricted flow passage includes at least one hole throughthe piston, and the valve means comprises a valve member adapted to seatover the end of the hole when the piston rod moves at more than apredetermined speed into the cylinder.

6. A shock absorbing and energy dissipating device as claimed in claim 5wherein the valve member has at least one flow-restricting orificeco-operable with the hole through the piston when seated over the hole.

7. A shock absorbing and energy dissipating device as claimed in claimwherein the hole through the piston is co-operable with a second valvemember adapted to seat over the other end of said hole when the pistonrod moves at more than a predetermined speed into the cylinder.

8. A shock absorbing and energy dissipating device as claimed in claim 7wherein the second valve member has at least one flow-restrictingorifice co-operable with the hole through the piston when seated there-9. A shock absorbing and energy dissipating device as claimed in claim 1wherein the main container is a cylinder and the plunger is a pistonrod, a piston being formed at the inner end of the piston rod toslidably engage within the cylinder, the second container being inunrestricted connection with the zone defined between the piston, thepiston rod, the cylinder and the seal, said zone forming part of theunrestricted flow passage.

10. A shock absorbing and energy dissipating device as claimed in claim1 wherein the second container is formed internally within the plunger.

11. A shock absorbing and energy dissipating device as claimed in clain1, the dimensions of the plunger being such taht the total reduction influid volume within the device when the plunger fully enters the maincontainer is greater than the volume of gas in the second container inthe fully extended position of the plunger.

12. A shock absorbing and energy dissipating device as claimed in claim1 wherein the total fluid volume in the fully-extended position of thedevice does not exceed five times the reduction of total fluid volumecaused by entry of the piston rod into thelcylinder during movement ofthe rod from the fully-extended position to the fully-contractedposition.

' 'UNITED STATES PATENT OFFICE I CERTIFICATE OF CORRECTION Patent No.3,7 l-7,9 3 Dated July 4-, 973

Inventor(s Ralph Savery It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Claim 2 line 2, change "claim 1" to -claim 9 Claim 12, line 2, change"claim 1 to -claim 9--.

Signed and sealed this 8th day of January 1971;.

SEAL) Attest:

EDWARD M.ELETCHER,JR. RENE D. I TEGTMEYER Attesting Officer ActingCommissioner of Patents USCOMM-DC 603764 69 FORM F-O-1050 (10-69) I Q vQ U. 5. GOVERNMENT PRINTING OFFICE 2 l9? 0-366-334.

1. A shock absorbing and energy dissipating device suitable for anaircraft landing gear comprising: i. a main container having a singleaperture intended to be uppermost in use ii. a plunger movable into andout of the aperture in the main container iii. a seal at the saidaperture to sealingly engage against the relatively movable surface ofthe plunger iv. a volume of liquid within the main container v. a secondcontainer containing gas at pressure vi. a restricted flow passagebetween the main container and the second container for restrictingliquid flow between them vii. an unrestricted flow passage forunrestricted gas flow in a generally upward direction from substantiallythe whole volume of the main container to the second container open whenthe plunger is in its fully extended position.
 2. A shock absorbing andenergy dissipating device as claimed in claim 1 wherein the piston rodis hollow and part thereof forms the second container.
 3. Ashock-absoring and energy dissipating device as claimed in claim 2wherein the piston rod has abutment means adapted to determine theoutermost position of the piston rod at which the piston is spaced fromthe said seal, the piston rod between the piston and the seal havingaxially-spaced holes each connecting the hollow piston rod to thecylinder at least the hole nearer to the seal forming part of saidunrestricted flow passage.
 4. A shock absorbing and energy dissipatingdevice as claimed in claim 1 including valve means responsive tomovement of the plunger to close the said unrestricted passage.
 5. Ashock absorbing and energy dissipating device as cliamed in claim 4wherein the unrestricted flow passage includes at least one hole throughthe piston, and the valve means comprises a valve member adapted to seatover the end of the hole when the piston rod moves at more than apredetermined speed into the cylinder.
 6. A shock absorbing and energydissipating device as claimed in claim 5 wherein the valve member has atleast one flow-restricting orifice co-operable with the hole through thepiston when seated over the hole.
 7. A shock absorbing and energydissipating device as claimed in claim 5 wherein the hole through thepiston is co-operable with a second valve member adapted to seat overthe other end of said hole when the piston rod moves at more than apredetermined speed into the cylinder.
 8. A shock absorbing and energydissipating device as claimed in claim 7 wherein the second valve memberhas at least one flow-restricting orifice co-operable with the holethrough the piston when seated thereover.
 9. A shock absorbing andenergy dissipating device as claimed in claim 1 wherein the maincontainer is a cylinder and the plunger is a piston rod, a piston beingformed at the inner end of the piston rod to slidably engage within thecylinder, the second container being in unrestricted connection with thezone defined between the piston, the piston rod, the cylinder and theseal, said zone forming part of the unrestricted flow passage.
 10. Ashock absorbing and energy dissipating device as claimed in claim 1wherein the second container is formed internally within the plunger.11. A shock absorbing and energy dissipating device as claimed in clain1, the dimensions of the plunger being such taht the total reduction influid volume within the device when the plunger fully enters the maincontainer is greater thAn the volume of gas in the second container inthe fully extended position of the plunger.
 12. A shock absorbing andenergy dissipating device as claimed in claim 1 wherein the total fluidvolume in the fully-extended position of the device does not exceed fivetimes the reduction of total fluid volume caused by entry of the pistonrod into the cylinder during movement of the rod from the fully-extendedposition to the fully-contracted position.